1. Field of the Invention
The present invention relates to an optical device equipped with a wavelength conversion function (hereinafter referred simply to “the optical device”), a laser apparatus which uses the optical device, and an exposure apparatus which uses the optical device.
2. Description of the Related Art
A number of proposals have been made as for an optical device which uses a ferroelectric crystal substrate of a ferroelectric crystal to perform wavelength conversion, light modulation, etc., and as for an apparatus comprising the optical device. JP-A-2009-31732 for instance discloses an apparatus which performs exposure processing using an optical device which performs light modulation. The apparatus comprises a light source which is formed by a semiconductor laser or the like to emit a light beam of a predetermined wavelength. The laser light from the light source, after getting shaped into a parallel light beam by an illumination optical system, impinges upon the optical device (spatial light modulator). In this optical device, a ferroelectric crystal is provided in the form of thin plate or slab, and a plurality of electrode elements are arranged at constant intervals on one major surface or the both major surfaces of the ferroelectric crystal, thereby forming a grid electrode. As an electric potential difference is applied between the electrode elements, an electric field develops in the ferroelectric crystal, whereby the refractive index periodically changes in the ferroelectric crystal and a diffraction grating is formed. Light is made incident upon the ferroelectric crystal in an approximately parallel direction to the longitudinal direction of the grid electrode so that Raman-Nath diffraction is primarily created, which realizes spatial light modulation.
It is beneficial to use wavelength conversion techniques which have been proposed for the purpose of reducing the size of such an apparatus and securing a proper wavelength. The technique according to WO2009/016709 for example proposes providing a wavelength conversion part in a ferroelectric crystal substrate to consequently shorten the wavelength of near-infrared laser light. This conventional technique uses an optical device in which a waveguide second harmonic generation (SHG) part is formed in a ferroelectric crystal substrate. In short, a fundamental emitted from a solid laser element is allowed to propagate to the second harmonic generation part provided in the optical device, and a wavelength-converted second harmonic is obtained which has half the wavelength of the fundamental (i.e., which has double the frequency of the fundamental) owing wavelength conversion. Therefore, application of the laser apparatus described in WO2009/016709 to the exposure apparatus (optical head) described in JP-A-2009-31732 makes it possible to reduce the apparatus size.
Further proposal has been made to additionally dispose an optical device which comprises a sum frequency generation (SFG) part subsequent to the optical device which comprises the second harmonic generation part in an attempt to shorten the wavelength of the laser light for use in the exposure apparatus.
However, whichever it is between when two or more wavelength conversion optical devices are combined to emit light at a predetermined wavelength and when a wavelength conversion optical device and light modulation optical device are combined, as these optical devices are all discrete elements, their optical axes must be aligned to each other at a high accuracy. A change in the temperature environment, a humidity variation, and heat developed in the optical devices, etc. could deviate the optical axes alignment, which then could destabilize the optical performances such as the light amount stability, the efficiency of wavelength conversion and the efficiency of light modulation.